The Prime Minister has repeatedly denied doing any backroom deals to get the proposals through and did so again in the Commons today when questioned by Mr Cameron.

The Tory leader then raised Mr Hoon's letter and told him: 'Don't take people for fools. Tell us the truth. What did he mean?'

Mr Brown responded: 'He was thanking the chairman of the Home Affairs Select Committee for doing exactly the right thing.'

David Davis, the former shadow home secretary who resigned as an MP after the 42-day vote to fight a by-election on a civil liberties platform, called on Mr Brown to explain the letter.

'This is yet further evidence demonstrating the grubby deal-making it took for Gordon Brown to get 42 days through the House of Commons, despite widespread opposition across the political parties,' he said.

'It is now incumbent upon Mr Brown to immediately explain precisely what his Chief Whip meant when he said the chairman of the Home Affairs Select Committee, Keith Vaz, would be "appropriately rewarded" for reversing his opposition to 42 days, and voting with the Government.'

A spokesman for Mr Hoon said: 'We would deny very strongly the suggestion that he's somehow been given something in return for his vote on 42 days.

'This is a private note between two old friends that included a light-hearted jokey remark. It's obvious it was a joke.'